1 1/2 pounds mixed citrus fruit (grapefruit, orange, lemon)
12 cups cold water
6 cups granulated sugar
Wash, dry, quarter, and seed fruit. Slice fruit quarters very thin. Measure 4 cups fruit and juice into an 8-quart heavy, enamel Dutch oven. Add water. Cover and let stand for 24 hours.
Uncover and boil gently until reduced to 6 cups, 2 to 2 1/2 hours.
Measure 6 cups pulp and juice into same Dutch oven. Add sugar. Bring to boil; stir to dissolve sugar. Boil rapidly; stir often, until mixture sheets off spoon (8 degrees F above the boiling point of water on candy thermometer).
Remove from heat. Stir gently and skim foam from top.
Ladle into hot, sterilized half-pint canning jars. Leave 1/4-inch headspace. Seal with canning lids according to manufacturer's directions.
In a boiling water bath, process for 5 minutes from the moment water returns to boil. Add 1 minute for each 1,000 feet above sea level.
Cool on rack.
Store in a cool, dry, dark place.